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Abstract

Optical microresonator biosensors have proven to be a valid tool to perform affinity analysis of a biological binding event. However, when these microresonators are excited with a single optical mode they can not distinguish between a thin dense layer of biomolecules or a thick sparse layer. This means the sensor is ”blind” to changes in shape of bound biomolecules. We succeeded in exciting a Silicon-on-Insulator (SOI) microring with TE and TM polarisations simultaneously by using an asymmetrical directional coupler and as such were able to seperately determine the thickness and the density (or refractive index) of a bound biolayer. A proof-of-concept is given by determining both parameters of deposited dielectric layers and by analysing the conformational changes of Bovine Serum Albumin (BSA) proteins due to a change in pH of the buffer.

U.S. Department of Health and Human Services: Food and Drug Administration, “Innovation or stagnation: Challenge and opportunity on the critical path to new medical products” (2004). http://www.fda.gov/ScienceResearch/SpecialTopics/CriticalPathInitiative/ucm076689.htm .

U.S. Department of Health and Human Services: Food and Drug Administration, “Innovation or stagnation: Challenge and opportunity on the critical path to new medical products” (2004). http://www.fda.gov/ScienceResearch/SpecialTopics/CriticalPathInitiative/ucm076689.htm .

Figures (11)

Sensitivity of the microring resonator to binding of a thin biolayer in water for fundamental quasi-TE mode and fundamental quasi-TM mode, obtained with Fimmwave. The height of the waveguide is fixed at 220 nm and the excitation wavelength is 1550 nm. The region of interest is denoted by W1 and W2, where only the fundamental TE and the TM mode are guided

(a) View of the cross section of the ring waveguide as it is used for simulations. (b) Simulations of the wavelength shifts for both modes in function of the thickness of the protein layer for various refractive indices of this layer. The fitting of this data to the model results in an R2 value of 0.9998.

(a) Effective index of the first three guided modes for a rectangular waveguide with a height of 220 nm and water cladding. The black lines show the slight phase mismatch for a 550 nm ring waveguide and a 290 nm access waveguide. (b) Measured fiber to fiber spectrum of the microring with water cladding. Both the TE and the TM resonances are visible

SEM image of the microring with access waveguide and a square region where the ions bombarded the coupling section (a). After the ion bombardment, a SEM image of the cross section of the coupling section is taken (b), which shows the waveguide dimensions.

Schematic diagram of the calibration and the actual experiment. The calibration measurements are done before the measurements for the actual experiment start. They are used to obtain the simulation parameters. These simulations finally determine the model to solve the experimental measurements to the characteristics of the protein layer.

By measuring the FSR of both modes during a water phase prior to the following BSA experiment, the (a) width of the ring waveguide was determined with a mean value of 491.7 nm. (b) The height was calculated to be 210.7 nm.

(a) Simulation of the wavelength shift of the TE mode in function of the refractive index of a cladding layer with waveguide dimensions of W = 491 nm and H = 210 nm. The wavelength shift is simulated with respect to a buffer with nbuff = 1.32. (b) Shift of the TE resonance when switching from water to buffer (PBS in this case), as measured in the BSA experiment.

Tables (1)

Table 1 Comparison of the thickness, refractive index and adsorbed mass between the technique described in this paper (SOI Microring) and the silicon nitride dual polarisation interferometric (DPI) technique used in [22].

Metrics

Table 1

Comparison of the thickness, refractive index and adsorbed mass between the technique described in this paper (SOI Microring) and the silicon nitride dual polarisation interferometric (DPI) technique used in [22].